Microwave maser materials consisting of chromium doped sodium indium tungstate and iron doped sodium indium tungstate



Sept. 8, 1964 I n. R. soDEN 3,148,149 urcaoma uAsER MATERIALS cousrsrmc oF canonruu vom somma mmm Tuncsma AND IRON DOPED SODIUM INDIUII TUNGSTATE Filud Dec. 8, 1961 INVENTOR By R. SODEN ATTO United States Patent() CONSISTING phone Laboratories, Incorporated, New York, N.Y., a

corporation of New York Filed Dec. 8, 1961, Ser. No. 158,013 2 Claims. (Cl. 252-625) This invention relates to single crystal tungstate materials containing small additions of paramagnetic ions and to devices utilizing such crystals.

In recent years a new amplification principle, that of plification by stimulated emission of radiafor November/December 1958, and the article entitled A Solid-State Maser, a Supercooled Amplifier, page 16 of Electronics, Engineering Edition, April 25, 1958.

Among the more promising microwave maser macontaining small amounts of paramagnetic ion from which the stimulated emission occurs. Due to the iniiuence of the internal electricalv field of the hos-t crystal, the malishing the requisite nonequilibrium electron population distribution in a pair of the energy levels and subsequently amplifying a signal of a frequency which satisfies Plancks Law with respect to the two energy levels in nonequilibrium is understood by the art.

Among the more successful paramagnetc ions used to stimulate emission are iron and zero field splitting, the range of signal frequencies amplified by these ions is dependent upon the range of zero field splittings exhibited by the ions. Unfortunately, the range of splittings is limited due to the relatively few ost lattices capable of accepting the ions. Such lattices amplication of large signal frequencies. larly, it has been found that a monovalent-trivalent metal ion tungstate of the composition Na 5In0 5WO4, in which Aiumiiiiizi-iiff eryl 3,148,149 Patented Sept. 8, 1964 TABLE 1 Zero Field Splitting in kmc. Host Lattice Chromium Sodium-indium tungstate Rutile The large zero field splittings realized by the chromium and iron ions in the host lattice of the 'invention are conducive to the amplifying of signal frequencies up to approximately kmc. Signal amplification up to approximately 61 kmc. is associated with the 53.6 kmc. zero field splitting of emerald. The progressively smaller splittings associated with the rutile and aluminum oxidelattices cause a corresponding decrease in emission frequency.

The invention may be more easily understood by reference to the drawing in which the figure is a sectional view of an apparatus utilizing the composition of the invention. Referring more particularly to the figure, there is'shown an illustrative microwave maser device in which amplification of an input signal takes place by stimulated emission of radiation from the tungstate crystals of the invention. This device is described in detail in the 1958 November/December issue of The Microwave Journal pages 19 and 20. Briefiy, a crystal 1 of the invention is located in cavity 2, designed to support microwave energy at two different frequencies, one being the pump frequency 3 and the other being the signal frequency 4. The crystal is acted upon by a direct-current magnetic field produced by pole pieces 5 and by two RF magnetic fields associated with the two frequencies 3 and 4 produced by means not shown. The cavity 2 and associated waveguides 6, which couple the two RF energies into the cavity tube, are immersed in a liquid helium bath 7 which is contained in a Dewar flask 8. Flask 8, in turn, is immersed in a liquid nitrogen bath 9 which is contained in a Dewar flask 10. The circulator 11 is a fourterminal pair device with a nonreciprocal property indi-A cated by its symbol. A signal from antenna 12 is sent to cavity 2. The amplified signal from cavity 2 is sent to receiver 13. Any reflected signal from the receiver 13 is directed to a dummy load 14, where it is absorbed.

In one illustrative embodiment, a D.C. magnetic field of 10,000 gauss and an RF pumping frequency of 95.I

kmc. is utilized in conjunction with a chromium-doped sodium-indium tungstate crystal ofthe invention to amplify a 50 kmc. signal.

As understood by the art, although in principle there is no lower limit on the concentration of the chromium or iron ions utilized, a practical limit of about 0.01 atom percent iron or chromium in place of the trivalent indium ion of the host lattice is imposed by the necessity of having sufficient unpaired electrons available in the negative temperature state to adequately -amplify the input signal. The optimum concentrations for maximum amplification in general are o-f the order of from 0.05 to 0.5 percent for the materials of the invention. Ampliiication is obtained with higher concentrations above this level to a maximum of about 5 percent. However, the accompanying line broadening effects detract from the magnitude of amplification of the input signal. Such increased concentrations are useful for devices requiring a substantially increased band width.

It has been further determined that both chromium and iron in the 3+ valency state can be incorporated in the sodium-indium tungstate host lattice with a resulting improvement in maser operation. In such double incorporation, chromium is the principal active paramagnetic ion, with iron being employed to shorten the recovery time ofthe chromium electrons. As is understood by the art, it is advantageous to have the relaxation time of electron transitions from the lowest level of the nonequilibrium pair ot spaced energy levels to the bottom level of the selected three-level energy system as short as possible. A short recovery time permits more electrons to become available for pumping to the upper level of the nonequilibrium pair of spaced energy levels, thereby maximizing the eiciency of maser action. As is further understood by the art, such relaxation can be shortened by incorporating in the host lattice a second pararnagnetic ion. This second, or nonamplifying, ion is chosen such that, due to a mechanism of energy exchange with the principal active paramagnetic ion, the recovery time of the principal active ion is shortened. The mechanisms of this effect are described in detail in Patent No. 2,981,894, issued April 25, 1961.

When trivalent iron is utilized in combination with trivalent chromium in this fashion, a practical minimum limit of about 0.1 atom percent iron in place of the trivalent indium ion of the host lattice is imposed by the necessity of having a sufficient concentration of iron ions available for interaction with the chromium ions. An optimum iron concentration is in the order of one to tive percent. Enhancement of the recovery time of the chromium electrons increases with increasing concentration of iron ions above this level to a maximum of about l atom percent. Beyond this limit, interactions between the chromium and the iron ions result in a signicant increase in chromium line broadening.

The tungstate compositions of the invention are con veniently made by conventional methods known to the art. Such methods include the iiux method disclosed in United States Patent 3,003,142, issued November 3, 1961 and the Czochralski method described in an. article by I. Czochralski in Zeitschrift fur Physikalische Chemie, volume 92, pages 219 to 221 (1918).

A recent description of the Czochralski method is found in an `article by K. Nassau and L. G. Van Uitert in Journal of Applied Physics, volume 3l, page 1508 (1960). In accordance with this method, a melt is formed of a mixture of initial components, the composition of the melt being the desired composition of the grown crystal. A seed crystal is inserted into the top surface of the melt and simultaneously rotated and slowly withdrawn from the melt.

In accordance with the ilux method, a flux of sodium tungstate is utilized to promote crystal growth, the so dium ions, being incorporated into the crystalline lattice during such growth. Specilic examples of compositions grown by this method vare given below. These examples are to be construed as illustrative only and not as limiting in any manner the scope and spirit of ythe invention.

Example 1 255.7 grams Na2WO4, 214.52 grams W03, 20.8 grams In203, and 0.0228 gram Cr203 were dry mixed together. The mixture was then heated in a platinum crucible in air for 16 hours at temperature of 1120 C. The molten solution so formed was then cooled in air at a controlled rate of 2.5 C. per hour to a temperature of 700 C. The resulting solids were then furnace cooled to room ternperature and washed with hot sodium hydroxide, leaving sodium-indium tungstate crystals doped with trivalent chromium. The formed sodium-.indium tungstate crystals contained about 0.1 percent chromium in place of indium.

Example 2 255.7 grams Na2WO4, 214.52 grams W03, 20.8 grams In203, and 0.0239 gram FezOs were dry mixed. The mixture then underwent the same processing as detailed above, with the resulting formation of sodium-indium tungstate crystals doped with trivalent iron. The formed sodium-indium tungstate crystals contained about 0.1 percent iron in place of indium.

Example 3 255.7 grams Na2WO4, 1214.52 grams W03, 20.8 grams In203, 0.0228 gram C1303, .and 0.12 gram Fe203 were dry mixed. The mixture then underwent the same I' processing as deta-iled above, with the resulting formation of sodium-indium tungstate crystals doped with trivalent iron and trivalent chromium. The formed sodiumindium tungstate crystals contained about 0.1 percent chromium and 0.5 percent iron in place of indium.

What is claimed is:

1. A composition of matter consisting essentially of a single crystal sodium-indium tungstate material of the composition Na0 5In0,5WO4 in which from about 0.01 to 5 percent of the trivalent indium ions have been replaced by at least one trivalent ion selected from the group consisting of chromium and iron, but where both iron and chromium are incorporated into the material, from about 0.1 to l0 percent of the indium ions are replaced by iron ions and from about 0.01 to 5 percent of the indium ions are replaced by chromium ions.

2. A composition of matter in accordance with claim l in which from about 0.05 to 0.5 percent of the trivalent indium ions have been replaced by at least one trivalent ion selected from the group consisting of chromium and iron, but where both iron and chromium are incorporated into the material, from about l to 5 percent of the indium ions are replaced by iron ions and from about 0.05 to 0.5 percent of the indium ions are replaced by chromium ions.

References Cited in the file of this patent UNITED STATES PATENTS 3,003,112 Van Uitert Oct. 3, 1.961 3,015,072 Schulz-Du Bois et al. Dec. 26, 1961 

1. A COMPOSITION OF MATTER CONSISTING ESSENTIALLY OF A SINGLE CRYSTAL SODIUM-INDIUM TUNGSTATE MATERIAL OF THE COMPOSITION NA0.5IN0.5 WO4 IN WHICH FROM ABOUT 0.01 TO 5 PERCENT OF THE TRIVALENT INDIUM IONS HAVE BEEN REPLACED BY AT LEAST ONE TRIVALENT ION SELECTED FROM THE GROUP CONSISTING OF CHROMIUM AND IRON, BUT WHERE BOTH IRON AND CHROMIUM ARE INCORPORATED INTO THE MATERIAL, FROM ABOUT 0.1 TO 10 PERCENT OF THE INDIUM IONS ARE REPLACED BY IRON IONS AND FROM ABOUT 0.01 TO 5 PERCENT OF THE INDIUM IONS ARE REPLACED BY CHROMIUM IONS. 